South Korea raised its military watch alert to "vital threat" Wednesday before an expected North Korean missile launch, as UN chief Ban Ki-moon warned theKorean peninsula may be slipping out of control.

The North last week, told foreign diplomats in Pyongyang they had until April 10 to consider evacuation, fuelling speculation a launch was planned between Wednesday and the April 15 birthday celebrations for late founder Kim Il-Sung.

South Korean intelligence says the North has prepared two mid-range missiles for imminent launch from its east coast, despite warnings from ally China to avoid provocative moves at a time of soaring military tensions.